The present invention relates to plasma processing systems. In particular, the present invention relates to plasma processing systems having capability of current control for controlling plasma characteristics (such as one or more of plasma density, plasma uniformity, etc.).
Plasma processing systems, such as inductively coupled plasma (ICP) systems and transformer coupled plasma (TCP) systems, are employed in various industries for fabricating devices on wafers. For example, the industries may include semiconductor, magnetic read/write and storage, optical system, and micro-electromechanical system (MEMS) industries. A plasma processing system may generate and sustain plasma in a plasma processing chamber to perform etching and/or deposition on a wafer such that device features may be formed on the wafer. In fabricating devices, it may be important to control plasma characteristics specified in particular recipes in order to satisfy certain production yield requirements and/or certain feature specifications.
Typically, plasma control may involve controlling the power delivered by the power generator (e.g., a radio frequency power generator, or RF generator) of a plasma processing system. In generally, there may be two typical power control modes: the forward power mode and the delivered power mode.
In the forward power mode, the RF generator may assume, for example, a 50 ohm load at the input of the match network of the plasma processing system. When the match network is tuned to 50 ohm, the power delivered to the input of the match network may be the same as the forward power. However, if the match network is not tuned to 50 ohm, there may be a reflected power, and thus the power delivered to the input of the match network may not be the same as the forward power.
In the delivered power mode, the RF generator may deliver the required power to the input of the match regardless of the impedance at the input of the match network. In the delivered power mode, there may be reflected power if the impedance at the input of the match network is not tuned to a predetermined value, for example, 50 ohm. If the impedance at the input of the match is tuned to 50 ohm, i.e., in matched condition and steady state, there may be no differences between the forward power mode and the delivered power mode.
In a typical power control mode, e.g., the forward power mode or the delivered power mode, the RF system of the plasma processing system may operate in an open loop utilizing predetermined power input, without responding to changes in important parameters. For example, the impedance at the output of the match network may change. The changes may be resulted from, for example, material deposition, component deformation, temperature changes during plasma processing, etc. in the plasma processing chamber. The changes may also be caused by the hardware layout variations from a plasma processing chamber to a different plasma processing chamber. As a result, even if the accuracy of the power of the RF generator and the accuracy of the reflected power of the match network are satisfactorily maintained, the changes of impedance at the output of the match network may cause substantial variations in the RF current(s) in the source coil(s) and may therefore cause significant variations in plasma characteristics, and thus variations on processing results on the wafers.
For achieving and maintaining the desirable plasma characteristics, even if the same recipe is employed, re-calibration may be required between process runs for processing different batches of wafers, and process re-qualification may be required when replacing components in a plasma processing system or when utilizing different plasma processing systems. Both re-calibration and process re-qualification may significantly reduce productivity and may significantly incur costs in manufacturing devices.
Attempts may be made to measure the power delivered to the output of the match network, for providing feedback to implement closed-loop power control. In general, measuring the power may require measuring the voltage, the current, and the phase angle between the voltage and the current associated with the power. A relatively small error in measuring the phase angle may result in a very significant error in the power measurement. For implementing the closed-loop power control, sophisticated sensors or measurement mechanisms with high accuracy levels may be required. As a result, substantial costs may be incurred.